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Investments and Fair Value Measurements - Convertible senior notes (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
Available for sale securities:  
Available-for-sale securities sold $ 0
Maturity period for marketable securities  
Maximum contractual maturity period 2 years
Weighted average contractual maturity period 6 months
Fair value transfers  
Fair value of assets transferred from Level 1 to Level 2 $ 0
Fair value of assets transferred from Level 2 to Level 1 0
Fair value of liabilities transferred from Level 1 to Level 2 0
Fair value of liabilities transferred from Level 2 to Level 1 0
Unrealized losses  
Net unrealized losses 0
Convertible senior notes due 2023  
Available for sale securities:  
Proceeds from issuance of debt $ 230,000
Interest rate (as a percent) 3.25%
Notes fair value $ 234,000